This invention relates to a semiconductor device having a transistor structure, and more particularly to a semiconductor device having a charge accumulating layer under a gate electrode.
Recently, a floating-gate memory device has been proposed as a structure in which the "single electron phenomenon" is applied to a MOS semiconductor device (IBM, S. Tiwari, IEDM95, p. 521). In this device, a record, which indicates whether or not electrons flowing through the channel of the substrate are accumulated in semiconductor particles in the gate oxide film, will appear in current/voltage characteristics between its source and drain. Therefore, this device is expected to be applied as a memory device.
FIG. 1 is a sectional view illustrating the device structure proposed in the above document. In the figure, reference numeral 1 denotes an Si substrate, reference numeral 2 a tunnel oxide film, reference numeral 4 an SiO.sub.2 film, reference numeral 5 a gate electrode, reference numeral 6 a source region, reference numeral 7 a drain region, reference numeral 8 an inversion layer, and reference numeral 21 Si fine particles. This device is characterized in that Si fine particles 21 with a size of about 5 nm are provided on the tunnel oxide film 2 with a thickness of 2 nm or less, and the gate electrode 5 is provided on the resultant structure.
In this device, electrons contained in the inversion layer 8 directly tunnel into the Si fine particles 21 on the tunnel oxide film 2 when a gate voltage has been applied. When electrons have tunneled into the Si fine particles, the electron distribution of a conduction band in the inversion layer 8 located under the Si fine particles 21 varies. As a result, the threshold voltage of a gate having its channel connected to the band changes. Since the change in the threshold voltage is about 0.36V, the state of electrons in the Si fine particles 21 can be sensed by sensing a change in the current flowing through the inversion layer 8 with respect to the gate voltage.
FIGS. 2A, 2B and 2C are views illustrating changes in the conduction band of the above-described device. When a positive voltage has been applied to the gate, electricity is transmitted and accumulated into the Si fine particles from the inversion layer 8 via the tunnel oxide film 2, as is shown in FIG. 2A ("Write" state). Even if the application of the voltage to the gate electrode 5 is stopped, the electricity is retained in the Si fine particles 21, as is shown in FIG. 2B ("Store" state). In this state, the threshold voltage of the transistor increases. On the other hand, when a negative voltage has been applied to the gate, the electricity accumulated in the Si fine particles is discharged to the substrate side via the tunnel oxide film 2, as is shown in FIG. 2C. In this state, the threshold voltage returns to its original value ("Erase" state).
As described above, electricity can be transmitted into, retained in and discharged from the Si fine particles 21, and the threshold voltage of the device varies depending upon whether or not electricity is accumulated in the Si fine particles 21. This being so, this device can be used as a memory device.
Elements of this kind, however, have the following problems; in the conventional floating-gate device using the single electron phenomenon, the distance between a quantum dot and the channel layer of the substrate is as close as about 2 nm. Accordingly, the threshold voltage, which is disadvantageous, if high, in the case of a flash memory, can be suppressed. However, it is highly possible that electrons will return into the substrate. This being so, the retention time, i.e. the time required for the electricity in the quantum dot to discharge into the substrate side, is as short as several months. This is rather shorter than the retention time, i.e. several years, in the case of a usual flash memory which does not use the single electron effect.
Although as described above, the floating-gate device using the single electron effect can have a low threshold voltage but cannot retain electricity for a long time. This is a factor which makes it difficult to use the device as a memory device.